In photolithography, the wavelengths of radiation sources have progressed from the visible spectrum to the deep ultraviolet (approximately 365 nanometers to approximately 100 nanometers). The reduction in wavelength is dictated by the requirement for smaller circuit feature sizes and the particular wavelengths are determined by the availability of high power radiation sources.
For advanced photolithography, there is a need for short wavelength radiation sources to produce smaller and higher performance integrated circuits. Wavelengths of 157 or 126 nanometers in the deep ultraviolet spectrum and 13 or 10 nanometers in the soft x-ray spectrum (sometimes characterized as the extreme ultraviolet spectrum) are being considered for advanced photolithography systems. The presently known photolithography apparatus suffers deficiencies in producing high power radiation at such wavelengths at or below 157 nanometers in an efficient, reliable and economical fashion.
One example of a proposed soft x-ray or extreme ultraviolet projection lithography apparatus using an arc-shaped illumination field, called a "ring field" is described in Ceglio, Hawryluk and Sommargren, "Front-End Design Issues In Soft-X-Ray Projection Lithography," Applied Optics, vol. 32, pp. 7050-7056 (Dec. 1, 1993). Plasma emitting radiation in a desired wavelength is created by striking a target with an optical laser beam focused to a small spot. In one such system, it is proposed that the optical laser beam be scanned across the target in an arc or ring field pattern (i.e. creating a scanned point-type source). In another such system, an arc or ring field pattern is generated from a point-source of radiation by condenser optics, creating a narrow ring field. Ultimately a mask and wafer (typically coated with a photoresist) is illuminated with the arc or ring field pattern. Because the pattern does not illuminate the entire mask or wafer, the pattern is also scanned to illuminate the entire mask or wafer. One disadvantage is that scanning the laser beam to produce a ring field of extreme ultraviolet radiation increases the exposure time and generates inefficiencies and can result in a non-uniform field, which is not desirable in photolithography. Other disadvantages are that known ring field condenser optics are complex, difficult to properly align and expensive. Known condenser optics that use point-like radiation sources typically do not provide a sufficiently high amount of light and provide an undesirably high level of coherence for optimal mask illumination for photolithography applications.
There are also various techniques for shaping laser beams. For example, creating a line focus is known, as described in I. N. Ross et al., "Design and Performance of a New Line Focus Geometry For X-Ray Laser Experiments," Applied Optics, Vol. 25, No. 9, pp. 1584-87 (May 1, 1997).
Accordingly, there is a need for a system that provides a shaped illumination field, without resorting to scanning a series of points from a point source in creating arc shapes or relatively complex condenser optics in the creation of the shaped radiation field.